Retroreflective sheeting has the ability to redirect light incident upon a major surface of the sheeting toward its originating source. This unique ability has led to the wide-spread use of retroreflective sheeting in a wide variety of conspicuity enhancement applications relating to traffic and personal safety marking. Typical examples of uses of retroreflective sheeting include the placement of such sheeting on road signs, traffic cones, and barricades to enhance their conspicuity, particularly under poor lighting conditions such as, for example, night-time driving conditions or in conditions of inclement weather. These uses typically allow the sheeting to be adhered to relatively flat, rigid surfaces, thereby allowing the sheeting to be relatively inflexible. Additionally, signing applications are characterized by relatively predictable, standardized viewing geometries.
There are essentially two types of retroreflective sheeting: beaded sheeting and cube corner sheeting. Beaded sheeting employs a multitude of independent glass or ceramic microspheres to retroreflect incident light. From an optics perspective, beaded sheeting typically exhibits strong rotational symmetry and entrance angularity performance because of the symmetrical nature of the beads. However, beaded sheeting tends to exhibit relatively low brightness when compared to cube corner sheeting. Additionally, beaded sheeting typically exhibits relatively good flexibility because the beads are independent from one another.
Cube corner retroreflective sheeting typically employs an array of rigid, interconnected cube corner elements to retroreflect light incident on a major surface of the sheeting. The basic cube corner element, now well known in the retroreflective arts is a generally tetrahedral structure having three mutually substantially perpendicular lateral faces which intersect at a single reference point, or apex, and a base triangle opposite the apex. The symmetry axis, or optical axis, of the cube corner element is the axis which extends through the cube apex and trisects the internal space of the cube corner element. In conventional cube corner elements which have an equilateral base triangle, the optical axis of the cube corner element is perpendicular to the plane which contains the base triangle. In operation, light incident on the base of the cube corner element is reflected from each of the three lateral faces of the element and is redirected toward the light source. Retroreflective sheeting generally incorporates a structured surface including at least one array of cube corner reflective elements to enhance the visibility of an object. When compared with beaded sheeting, cube corner retroreflective sheeting exhibits relatively greater brightness in response to light incident at relatively low entrance angles (e.g. near normal light). However, cube corner retroreflective sheeting also exhibits relatively poor entrance angularity and rotational symmetry performance. Additionally, cube corner retroreflective sheeting is typically stiffer than beaded sheeting because the cube corner elements are all interconnected.
The optics of cube corner retroreflective sheeting may be designed to exhibit optimal performance at a specific orientation. This may be accomplished by forming the cube corner elements of the retroreflective sheeting such that their optical axes are canted relative to an axis perpendicular to the base plane of the sheeting. For example, U.S. Pat. No. 4,588,258 to Hoopman (the '258 patent) discloses retroreflective sheeting which employs optics having canted cube corner elements which form opposing matched pairs. The sheeting disclosed in the '258 exhibits a primary plane of improved retroreflective performance at high entrance angles, identified as the x-plane in the '258 patent, and a secondary plane of improved retroreflective performance at high entrance angles, identified as the y-plane in the '258 patent. In use, it is recommended that sheeting manufactured in accordance with the '258 patent be oriented such that its principal plane of improved retroreflective performance (e.g. the x-plane) is coincident with an expected entrance plane. Thus, sheeting in accordance with the '258 patent has a single preferred orientation.
Many conspicuity applications could benefit from a retroreflective sheeting which exhibits two primary planes of improved retroreflective performance at relatively high entrance angles. For example, some signing applications may benefit because a second primary plane of improved retroreflective performance at high entrance angles would provide a second preferred orientation for placing sheeting on road signs. A second preferred orientation should result in increased efficiency and reduced waste in the sign construction process.
A second application which could benefit from retroreflective sheeting having two primary planes of improved retroreflective performance at high entrance angles is the field of vehicle conspicuity marking, and especially the field of truck conspicuity marking. Many accidents involving trucks are side-on collisions which occur in poor lighting conditions because an oncoming vehicle cannot see a truck crossing its path in time to avoid the accident. Studies have shown that appropriate truck conspicuity marking programs can significantly reduce the incidence of such side-on collisions. See, e.g. Finster, Schmidt-Clausen, Optimum Identification of Trucks for Real Traffic Situations, Report on Research Project 1.9103 of the Federal Highways Agency, April, 1992. The United States has implemented a regulation relating to retroreflective conspicuity enhancement systems for commercial vehicles. It is known that other countries are pursuing relations governing full contour markings on long and heavy vehicles through the UN/ECE.
Full contour marking of commercial vehicles (e.g. marking the entire perimeter of a vehicle's side and/or rear walls) allows viewers to determine the full dimensions of the vehicle. However, full contour marking requires that retroreflective sheeting be placed in both a horizontal orientation (e.g. along the bottom and/or top of a vehicle) and in a vertical orientation (e.g. along the side of a vehicle). It would be desirable to provide a single retroreflective sheeting product which performs equally well in either orientation such that it could be placed on a vehicle in either a vertical or a horizontal orientation. The sheeting optics should provide strong retroreflective performance in two perpendicular planes. From a physical perspective, truck conspicuity applications require the sheeting to be adhered to the side of a vehicle which may include corrugations and/or protruding rivets or which may be made from a flexible tarpaulin. Accordingly, the sheeting should be able to conform to underlying surfaces which are irregular or flexible.